1. Field of the Invention
This application relates to methods of minimizing inhomogeneities in magnetic fields caused by superconductors placed in the magnetic fields. More particularly, it relates to a method of achieving a homogeneous field in a superconducting NMR probe coil.
2. Description of Related Art
The operating principle that underlies magnetic resonance instrumentation involves the application of a high-frequency alternating magnetic field to a sample in the presence of an orthogonal static magnetic field. This combination of fields causes nuclei in the sample to precess about the static field. The nuclear precession in turn creates an alternating magnetic field--the nuclei, in a sense, become radio transmitters. In modern nuclear magnetic resonance (NMR) spectrometers and magnetic resonance imaging (MRI) instruments, the static field is supplied by a superconducting magnet. For high field NMR spectroscopy, the static field at the sample is 5 to 18T. For high field MRI, the field maybe 1T.
The sensitivity of instrument depends upon the signal-to-noise ratio of the receiver. The sensitivity may be increased by increasing the signal (providing a higher static field) or by decreasing the noise. Noise may be due to sample noise or receiver noise. Where receiver noise is limiting, as it is in spectroscopy and in low field MRI, the signal-to-noise ratio may be greatly enhanced by lowering the noise of the receiver coil.
The use of superconductors, especially high temperature superconductors (HTS) for NMR and MRI coils has been previously reported. For example, Withers, et al., U.S. Pat. No. 5,276,398 describes a thin-film HTS probe for magnetic resonance imaging. Brey, et al., "Nuclear Magnetic Resonance Probe Coil" filed Jun. 5, 1995, describes HTS coils for NMR spectroscopy. Withers, U.S. Ser. No. 08/409,506 describes pairs of HTS coils for NMR spectroscopy. Kotsubo, U.S. Ser. No. 08/297,352 describes an apparatus for cooling NMR probe coils in a spectrometer. Each of these references in incorporated herein by reference.
While superconductors are an attractive material for NMR receiver coils due to their extremely low resistance, and correspondingly low noise levels, they have certain drawbacks. Superconductors exhibit perfect diamagnetism. When placed in a strong magnetic field, this large diamagnetic effect, combined with flux pinning can lead to unacceptable static field inhomogeneities in the environment of the receiver coil. Therefore, a method of avoiding static field inhomogeneities near superconducting receiver coils is required.